An organic EL (Electro Luminescence) device, which has a structure such that a cathode, an organic light emitting layer, and an anode are laminated, is a device such that a positive hole injected from the anode and an electron injected from the cathode are re-coupled at an organic light emitting layer, so that excitation energy is released as light emission.
The related art techniques are primarily of a bottom emission type such that a switching device such as a TFT (Thin Film Transistor), etc., is formed on a glass substrate; an organic EL device which has laminated an ITO (Indium Tin Oxide) electrode, an organic light emitting layer, and a metal cathode in that order is arranged adjacent to the switching device; and light is taken out through a transparent substrate.
There is a problem with the bottom emission type that the TFT and the organic EL device need to be lined up on the glass substrate, so that an aperture rate remains around 10-30%, as it is not possible to increase the aperture rate theoretically.
Thus, a top emission type is being zealously studied such that the TFT is covered with an interlayer insulator, on which an organic EL device is formed.
The top emission type has an advantage that the aperture rate may be made greater relative to the bottom emission type as the TFT and the organic EL device are connected through a contact hole provided at the interlayer insulator; on the interlayer insulator, individual layers are laminated in the order of the cathode, the organic light emitting layer, and an transmissive anode; and light is taken out through the transmissive anode on the top layer.
Moreover, low molecular type organic light emitting layer is formed by vacuum deposition method in the related art, and when the display size is larger, a low level of efficiency of use of material, generation of particles from a mask, and an alignment error of a shadow mask are severer. Therefore a high molecular type organic EL device such that a high molecular type organic light-emitting layer is formed by an inkjet method is attracting special attention.
In present inkjet devices, it is difficult to properly print a different high molecular organic light-emitting material of R, G, and B only on a predetermined cathode separately, so that a method is used of providing a partition (bank) around the cathode formed on the interlayer insulator, and making the partition water-repellent using CF4 plasma processing to control wettability, thus dropping the high molecular organic light-emitting material into an aperture within the partition.
In the top emission type, a contact hole is needed to connect the TFT and the organic EL device. When a cathode material is deposited, using sputtering and vacuum deposition, on the interlayer insulator on which the contact hole is provided, and the cathode is formed using photolithography, a dent is produced due to the contact hole on the cathode surface being directly above the contact hole. Thus, when the high molecular inorganic light-emitting material is printed on the cathode using the inkjet method, the film thickness of the organic light-emitting layer increases due to the dent of the cathode that is formed directly on top of the contact hole, so that uniformity of the film thickness is reduced. Thus, light emission becomes non-uniform within pixels surrounded by partitions, so that a good image quality is not obtained with the display device using the organic EL device.
In order to obtain good image quality, it is important to planarize the contact hole and the cathode. Thus, various proposals are being made in this regard.
For example, in Patent document 1, a contact hole is arranged to be provided beside a pixel electrode (a cathode), and a connecting wiring with the contact hole filled with an electro-conductive material is used to connect the TFT and the pixel electrode, and to have a part of a partition arranged on the contact hole.
In such a structure, the dent of the connecting wiring due to the contact hole is covered with the partition, so that it is separated from a light-emitting region, making it possible to form a flat cathode at least within an aperture surrounded by the partition.
Patent document 1 adopts a method of forming the partition having an aperture after forming a connecting wiring and a pixel electrode (cathode) in order to implement the above structure.
In a case of a low molecular organic EL device, a structure is already disclosed in Patent document 2 which provides a part of a partition on a contact hole. Moreover, in Patent document 3, a method is disclosed such that, a pixel electrode (cathode) is formed on the contact hole, then a dent of the pixel electrode is implanted with an insulator within a hole that has a liquid material filled in and cured, and then a partition is provided such that one part of the partition is arranged on the contact hole, or in other words, on the insulator within the contact hole. In the method of Patent document 3, the dent due to the contact hole is filled in with the insulator within the hole, so that a certain level of flatness is obtained, and the insulator within the hole is covered with the partition so that there is separation from a light-emitting region; therefore, the interior of the aperture surrounded by the partition becomes a flat cathode. In order to achieve the structure of Patent document 3, the method must be adopted which forms the partition having the aperture after forming the insulator within the hole and the pixel electrode (cathode).
A different method is disclosed in Patent document 4, for example. In Patent document 4, there are disclosed a first method of implanting a contact hole with an electro-conductive material, then flattening using polishing (a Damascene process), and then forming a pixel electrode (cathode) on the contact hole; and a second method of forming the pixel electrode on the contact hole, and then filling a dent produced on the pixel electrode with an insulating material such as silicon oxide, etc. Patent document 4 discloses that a method is adopted of forming a partition having an aperture on an interlayer insulator, and then forming a pixel electrode (cathode) in the aperture.
Moreover, compared to a photolithography method such that a resist pattern is formed on a film deposited by sputtering or vacuum deposition, an inkjet method is low cost, so that forming a high molecular organic EL device using the inkjet method contributes to a decreased cost of a display device. Moreover, a proposal is being made to seek a further decrease in cost by not only forming a high molecular organic EL device by the inkjet method, but also to form, by a printing method, a cathode formed by the photolithography method according to the related art.
For example, Patent document 5 discloses a method of applying by a printing method, as an electrode, a high molecule material, which is an electro-conductive fine particle such as indium, carbon, etc., dispersed. Patent document 6 discloses providing between a light-emitting layer and a main cathode, an electron injecting layer which includes metal complexes, applying, as the main cathode, PEDOT-PSS.
Various proposals are also made of a method of forming a contact hole by an inkjet method. For example, Patent document 7 discloses applying, by an inkjet method, a dot-shaped liquid repellant material to a location where a contact hole is to be formed to form a liquid-repellent section and then applying an insulating material around the liquid repellant section. The insulating material is repelled by the liquid repellant section to become a contact hole. Such a method as described above can be adopted to further decrease the cost of a top emission organic EL device.
However, Patent document 5 only discloses the structure of the organic EL device, and does not disclose the structure of the contact hole and a method of connecting with the TFT at all. Moreover, Patent document 6 discloses substrates using top emission type, but refers to related art cases which are all related to a bottom emission type, and does not disclose flattening of contact holes and cathodes at all.
Thus, when manufacturing the display device using the top emission type organic EL device, even when the cathode could be formed using Patent document 5 or 6, flattening of the contact hole and cathode that is important in top emission types is difficult, so that a method of flattening the contact hole and cathode of Patent documents 1, 3, and 4, etc., must be adopted.
Moreover, when the cathode is formed by the inkjet method, it is difficult to make the minimum space small because of curving of a head, meandering of the head, and ejection variation. In other words, a highly fine fabrication is difficult. In general, when forming the cathode by the inkjet method, the minimum space is 30-50 μm.
On the other hand, printing of a high molecular-type organic light-emitting material using the inkjet method uses a liquid-repellant partition to respond to the highly fine fabrication, so that, taking into account the process complexities, the method of using the previously-described partition is most desirable.
In the method as disclosed in Patent documents 1 and 3, the partition needs to be formed such as to cover the contact hole after forming the cathode and the connecting wiring on the contact hole, so that the partition for separating the high molecular organic light-emitting layer cannot be used when printing the cathode. Therefore, the minimum space of the cathode is determined by the resolution of the inkjet method. Thus, when the cathode is flattened by the methods disclosed in Patent documents 1 and 3, there is a disadvantage that the minimum space of the cathode cannot be reduced, so that the resolution of the cathode cannot be improved. In particular, in the method in Patent document 3, the process of forming the insulator within the hole is required, leading to an increased cost in the organic EL device.
Moreover, when flattening the contact hole and the cathode by the first method disclosed in Patent document 4, the electro-conductive material is filled into the contact hole, flattening is made by the Damascene method, then the partition is formed, and then the cathode is formed, so that it is possible to print the cathode by using the partition for separating the high molecular organic light-emitting layer; in other words, a highly fine fabrication of the cathode may be realized using the printing method. However, in order to flattering the contact hole by the Damascene method, a polishing machine, a machine for depositing an electro-conductive material to be filled, and a cleaning machine after the polishing is necessary. The polishing process causes dust emission therefrom so that separation from other areas becomes necessary, making an increased cost such as an additional clean room required, etc., so that it is difficult to manufacture the display device at low cost.
Moreover, when the cathode is flattened by the second method disclosed in Patent document 4, the insulating material which is filled in the dent of the cathode does not function as an electrode, so that the area of the cathode becomes substantially small, decreasing the uniformity within in the pixel area and the brightness of the display device. Moreover, a difference occurs in the density of electric currents which flow through individual organic EL devices, leading to decreasing the long-term reliability of the overall display device.
Furthermore, even when the contact hole is formed by a printing method by the method in Patent document 7, flattening of the contact hole and the cathode that is important in the top emission type cannot be realized, so that the method of flattening of the contact hole and the cathode of Patent documents 1, 3, and 4, etc., must be adopted. Moreover, in the method in Patent document 7, it is common for the insulating material not to stop at the edge of the liquid repellant section, and partially run onto the edge of the liquid repellant section, so that the bottom part of the contact hole is likely to become inversely tapered after removing the liquid repellant section. In particular, there is a disadvantage that, if the film thickness of the liquid repellant section is large, in case the contact hole is filled with the electro-conductive material, a void may occur in an inversely tapered portion, decreasing the long-term reliability of the contact hole.
As described above, in the top emission type organic EL device which forms the cathode and the high molecular organic light-emitting layer by the printing method, a technique of making the flattening of the cathode and the highly fine fabrication of the cathode compatible is yet to be proposed; thus, in order to realize, at low cost, a display device with a high aperture rate and a uniform image quality, the printing technique of the cathode that makes it possible to make the flattening of the cathode and the highly fine fabrication of the cathode compatible is becoming very important. Moreover, for a further decrease in cost, development of a printing method which makes it possible to form a contact hole having long-term reliability is also important.    Patent Document 1 JP2004-119219A    Patent Document 2 JP2001-148291A    Patent Document 3 JP2009-36948A    Patent Document 4 JP2005-197027A    Patent Document 5 JP2002-237389A    Patent Document 6 JP2005-79064A    Patent Document 7 JP2009-21552A